Although the red-bellied piranha, Pygocentrus nattereri is widely portrayed as a pack-hunting predator virtually nothing is known about its shoaling behaviour. Using fish collected in the flooded forest of the Brazilian Amazon, we conducted a range of choice tests to determine whether individual piranhas make non-random shoaling decisions. These revealed that partner size had a significant impact on shoaling preferences. Piranha shoals were size structured with large, sexually mature, individuals occupying the central zone (defined as within the polygon connecting fish at the edge of the shoal). Small, non-reproductive individuals showed greater motivation to feed. Fish did not discriminate between size-matched individuals from the same and different locality and we found no evidence of intraspecific aggression, despite the piranha's reputation for cannibalism.